The present invention relates to cobalt oxide particles, a process for producing the cobalt oxide particles, a cathode active material for a non-aqueous electrolyte secondary cell, a process for producing the cathode active material, and a non-aqueous electrolyte secondary cell. More particularly, the present invention relates to cobalt oxide particles useful as a precursor of a cathode active material for a non-aqueous electrolyte secondary cell which is capable of showing a stable crystal structure by insertion reaction therein, and producing a non-aqueous electrolyte secondary cell having a high safety and especially a high heat stability, a process for producing the cobalt oxide particles, a cathode active material for a non-aqueous electrolyte secondary cell using the cobalt oxide particles, a process for producing the cathode active material, and a non-aqueous electrolyte secondary cell using the cathode active material.
With the recent rapid development of portable and cordless electronic devices such as audio-visual (AV) devices and personal computers, there has been an increasingly demand for providing as a power source thereof, a secondary cell (lithium battery) having a small size, a light weight and a high energy density. Under this circumstance, lithium ion secondary cells have been especially noticed because of advantages such as high charge/discharge voltage as well as large charge/discharge capacity.
Hitherto, as cathode active materials useful for high energy-type lithium ion secondary cells having a 4V-grade voltage, there are generally known LiMn2O4 which has a spinel structure, LiMnO2 which has a corrugated layer structure, LiCoO2, LiCo1-xNixO2 and LiNiO2 which have a rock-salt layer structure, or the like. Among the secondary cells using these active materials, lithium ion secondary cells using LiCoO2 are more excellent because of high charge/discharge voltage and large charge/discharge capacity thereof. These lithium ion secondary cells have been required to show more excellent properties.
Specifically, when lithium ions are released from LiCoO2, the crystal structure of LiCoO2 undergoes Jahn-Teller distortion since Co3+ is converted into Co4+. When the amount of lithium ions released reaches 0.45, the crystal structure of LiCoO2 is transformed from hexagonal system into monoclinic system, and a further release of lithium ions causes the transformation of the crystal structure from monoclinic system into hexagonal system. Therefore, by repeating the charge/discharge reaction, the crystal structure of LiCoO2 tends to become unstable, resulting in release of oxygen from LiCoO2 and undesired reaction between LiCoO2 and an electrolyte solution.
Further, the reaction between LiCoO2 and the electrolyte solution is more active under higher temperature conditions. Therefore, in order to ensure safety of the secondary cell, it has been required to provide cathode active materials exhibiting a stable structure, namely a high heat stability even under high temperature conditions.
For these reasons, it has been required to provide lithium cobaltate (LiCoO2) exhibiting a stable crystal structure even when lithium is released therefrom.
Hitherto, in order to stabilize a crystal structure of lithium cobaltate and improve various properties thereof such as charge/discharge cycle characteristics, there are known a method of incorporating magnesium into lithium cobaltate particles (Japanese Patent No. 2797693 and Japanese Patent Application Laid-Open (KOKAI) Nos. 5-54889(1993), 6-168722(1994), 7-226201(1995), 11-102704(1999), 2000-12022, 2000-11993 and 2000-123834); a method of mixing magnesium-with lithium cobaltate particles by a hydrothermal synthesis method (Japanese Patent Application Laid-Open (KOKAI) No. 10-1316(1998)); a method of controlling a lattice constant of lithium cobaltate to improve properties thereof (Japanese Patent Application Laid-Open (KOKAI) No. 6-181062(1994)); or the like.
In addition, in order to obtain lithium cobaltate particles satisfying the above properties, cobalt oxide particles as a precursor thereof are also required to show an excellent reactivity. As the method for producing cobalt oxide particles having an excellent reactivity, there has been proposed a method of obtaining fine cobalt oxide particles by a wet reaction method (Japanese Patent Application Laid-Open (KOKAI) Nos. 10-324523(1998) and 2002-68750).
At present, it has been strongly required to provide cathode active materials satisfying the above requirements and cobalt oxide particles as a precursor thereof. However, such cathode active materials and cobalt oxide particles have not been obtained until now.
That is, in Japanese Patent No. 2797693 and Japanese Patent Application Laid-Open (KOKAI) Nos. 5-54889(1993), 6-168722(1994), 7-226201(1995), 11-102704(1999), 2000-12022, 2000-11993 and 2000-123834, there is described the method of obtaining lithium cobaltate particles containing magnesium by dry-mixing a cobalt compound, a lithium compound and a magnesium compound. However, in this method, since the obtained lithium cobaltate particles show a non-uniform distribution of magnesium within the particle, the crystal structure thereof tends to undergo destruction upon the release and insertion reactions of lithium ions. As a result, the lithium cobaltate particles fail to show an excellent heat stability.
In Japanese Patent Application Laid-Open (KOKAI) No. 10-1316(1998), there is described the method for producing lithium cobaltate particles by dispersing a cobalt compound and a magnesium compound in an aqueous lithium hydroxide solution and heat-treating the resultant dispersion. However, this method requires to conduct the hydrothermal treatment, and the obtained lithium cobaltate particles fail to show a small particle size and excellent particle properties.
Also, in Japanese Patent Application Laid-Open (KOKAI) No. 6-181062(1994), there is described lithium cobaltate having a c-axis length of lattice constant of not less than 14.05 Å. However, the obtained lithium cobaltate cannot be sufficiently improved in heat stability as compared to those containing magnesium and, therefore, fails to show an excellent heat stability.
Further, in Japanese Patent Application Laid-Open (KOKAI) Nos. 10-324523(1998) and 2002-68750, there is described the method for producing fine cobalt oxide particles by a wet reaction method. However, since no magnesium is contained in the obtained cobalt oxide particles, a cathode active material composed of lithium cobaltate particles produced from such cobalt oxide particles fails to show a sufficient heat stability.
As a result of the present inventors' earnest studies for solving the above problems, it has been found that by mixing a lithium compound with cobalt oxide particles containing magnesium therein which have a composition represented by the formula: (Co(1-x)Mgx)3O4 (0.001≦x<0.15), and have a BET specific surface area value of 0.5 to 50 m2/g and an average particle diameter of not more than 0.2 μm, or cobalt oxide particles surface-coated with magnesium hydroxide which have a composition represented by the formula: (1−x)Co3O4.3xMg(OH)2 (0.001≦x<0.15), and have a BET specific surface area value of 0.5 to 50 m2/g and an average particle diameter of not more than 0.2 μm; and heat-treating the resultant mixture, the obtained cathode active material has not only a more stable crystal structure but also a more excellent heat stability, and is useful as a cathode active material for a non-aqueous electrolyte secondary cell. The present invention has been attained based on the above finding.